Indium is used as a sputtering target material for forming light-absorbing layers for Cu—In—Ga—Se (CIGS) of thin-firm solar cells.
Indium sputtering targets are generally manufactured by melt casting, and some documents about that are known. For example, JPS 63-44820 B2 (Patent Document 1) discloses a method for manufacturing a flat plate-shaped sputtering target, which includes forming an indium thin film on a backing plate and then pouring indium on the thin film to cast it, so that an indium sputtering target is formed integrally with the backing plate.
In recent years, however, use of cylindrical sputtering targets instead of flat plate-shaped sputtering targets has progressed because the efficiency of use of such target materials is relatively high. A cylindrical sputtering target, which is made of a target material fixed around a backing tube (BT) as a core, can be rotated during sputtering. Thus, the entire surface of this target material can be used as an erosion region and uniformly sputtered, so that the resulting efficiency of use of the target material can be high.
Melt casting is also a main stream method for manufacturing cylindrical sputtering targets. For example, US 2003/0089482 A (Patent Document 2) discloses the following process. An inner tube (target holder) is made of a first material with a melting point of 900 K or higher, and then a cylindrical mold is concentrically placed so as to surround the inner tube. A second material with a melting point of 800 K or lower is poured in a molten state into a space between the mold and the inner tube. After cooling and solidification, the mold is removed, so that an outer tube (target) made of the second material is obtained.
It is also known that a bonding layer is placed between a backing tube (BT) and a target material so that the adhesion between them can be increased. For example, JP 2008-523251 A (Patent Document 3) discloses a tube-shaped target including a supporting tube, at least one target tube placed on the outer surface of the supporting tube, and a bonding layer placed between the target tube and the supporting tube, wherein the bonding layer is electrically conductive and has a bonding ratio of more than 90%.